Field of the Invention
The present invention relates to a positive electrode composition for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery. Particularly, the present invention relates to a positive electrode composition that can improve output power characteristics of a lithium ion secondary battery, and also can improve viscosity stability of a positive electrode slurry. The present invention also relates to a method for producing a positive electrode slurry having improved viscosity stability.
Description of the Related Art
With the progress in spread and miniaturization of mobile devices such as VTR, mobile phone and note PC, a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery has recently been used as a power supply therefore. Furthermore, in order to cope with a recent environmental problem, the nonaqueous electrolyte secondary battery has also attracted special interest as a power battery of an electric vehicle or the like.
Commonly, there has widely been used, as a positive electrode active material for a lithium ion secondary battery, LiCoO2 (lithium cobalt oxide) that can constitutes a 4 V-class secondary battery. When LiCoO2 is used as the positive electrode active material, it is put in practical use at a discharge capacity of about 160 mA/g.
Cobalt as a raw material of LiCoO2 is a scarce resource and is also unevenly distributed, which leads to high costs, and which may cause anxiety about supply of a raw material.
In response to these circumstances, LiNiO2 (lithium nickel oxide) also has been examined. Practically, LiNiO2 can realize a 4 V-class secondary battery having a discharge capacity of about 200 mA/g. However, there is a problem with stability of a crystal structure of a positive electrode active material upon charge and discharge.
Thus, there also has been made a study of realizing a discharge capacity at the same level as that of LiCoO2 at low cost while improving stability of a crystal structure by substituting nickel atoms of LiNiO2 with other elements. For example, it is considered that LiNi0.33Co0.33Mn0.33O2 is more advantageous than LiCoO2 in the respect of costs.
Furthermore, there has also been proposed LiNi0.5Co0.2Mn0.3O2 in which costs are reduced by decreasing the proportion of Co and a discharge capacity are improved by increasing the proportion of Ni. However, deterioration of output power characteristics is not commonly avoided when the proportion of Co decreases. Thus, there has been proposed a technology in which a disorder Ni atom arrangement in a crystal structure is reduced by making Li more excessive than a stoichiometric ratio, thereby compensating output power characteristics.    [Patent Document 1] JP-A-2007-188878    [Patent Document 2] JP-A-2002-075367    [Patent Document 3] JP-A-2000-106174    [Patent Document 4] JP-A-2003-142101
By the way, a positive electrode of a nonaqueous electrolyte secondary battery is formed by mixing a positive electrode active material with a binder such as polyvinylidene fluoride (PVDF) or N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry, and applying the positive electrode slurry to a current collector such as an aluminum foil. At this time, when lithium is released from the positive electrode active material, the lithium reacts with moisture contained in the binder to form lithium hydroxide. The thus formed lithium hydroxide reacts with the binder and thus the positive electrode slurry undergoes gelation, resulting in poor operability and a decrease in a yield. This tendency becomes remarkable when the proportion of lithium in the positive electrode active material is more excessive than a stoichiometric ratio and also the proportion of nickel is high.